So are there any gains, to buy it internally and sell on exchange ?

Yes there is, mosty on the Korean Upbit, but I believe its only for Korean citizens and you need to do KYC to trade there.

And who buys on exchanges and what do they do with these HBD, since withdrawal volume is low ?

Can't really tell, but probably speculators, who expect the price of HBD to go up.

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